Blue Emission in Sterically Shielded Multiresonant Thermally Ac-tivated Delayed Fluorescence Emitters

Multiresonant TADF materials are a promising class of emitters capable of addressing the BT.2020 industry requirement for blue emission in electroluminescent displays as they simulta-neously show narrowband emission and can harvest both singlet and triplet excitons to produce light. However, these emitters are typically planar and prone to aggregation and their moderately large singlet-triplet energy gap (∆EST) leads to slow upconversion kinetics resulting in severe ef-ficiency roll-off in the device. In this study we present a molecular design that simultaneously results in an emitter having a faster reverse intersystem crossing rate constant (kRISC) and sup-pressed aggregation in the film state. Mes-tDABNA emits at λPL of 465 nm as 4 wt% doped films in SF3-RZ and has a short delayed lifetime of 45.2 µs. Vacuum-deposited OLEDs with Mes-tDABNA showed blue emission at CIE coordinates of (0.13, 0.15) and a maximum external quantum efficiency, EQEmax, of 18.4%. Unsurprisingly, these devices suffered from rather strong efficiency roll-off (EQE1000 of 5.6%). With the aim of addressing this efficiency roll-off, hyperfluorescent devices containing DMAC-DPS as a TADF sensitizer were fabricated, which showed an improved EQEmax of 23.1% at CIE coordinates of (0.13, 0.17) and milder efficiency roll-off (EQE1000 of 12.7%). These devices showed one of the highest EQE1000 based on DAB-NA-based emitters to date.